The invention relates to a method for arranging a crystal to generate an oscillating signal for a circuit, in which method the crystal is loaded with capacitance in such a way that the crystal begins to oscillate at a desired frequency with a resonance circuit.
Further, the invention relates to an oscillating circuit comprising a crystal, and capacitors that load the crystal in such a way that the crystal oscillates at a desired frequency with a resonance circuit connected to the crystal.
Yet further, the invention relates to a mobile station comprising an oscillating circuit that is arranged to generate an oscillating signal required by the mobile station, whereby the oscillating circuit comprises a crystal, and capacitors that load the crystal in such a way that the crystal oscillates at a desired frequency with a resonance circuit connected to the crystal.
An oscillating signal for generating reference and clock frequencies is obtained by means of a crystal by loading the crystal capacitively. Seeing the correct capacitive load at its terminals, the crystal begins to oscillate at a desired frequency. A resonance circuit is connected in parallel with the crystal, whereby this resonance circuit is typically an integrated oscillating circuit. Thus, the crystal and the capacitors loading it are arranged outside the integrated circuit.
Particularly crystals in radio frequency use have typically no capacity to distribute the signal to several circuits. Further, the crystal is very sensitive to variation in the capacitive load, whereby the oscillating frequency of the crystal changes easily. At radio frequencies, the noise and jitter requirements of the oscillator are also extremely strict. Thus, a separate crystal is typically arranged for each circuit to generate an oscillating signal. The oscillating signal could be, in principle, taken not only from the resonance circuit connected in parallel with the crystal, but from a crystal terminal by means of a buffer, but the buffer causes impedance, which is seen at the crystal terminals. The change in the impedance caused by the buffer, in turn, changes the oscillating frequency of the crystal and, on the other hand, reduces the amplitude of the oscillation. In the worst case, the crystal load changes so much that the resonator circuit does not begin to oscillate at all.
U.S. Pat. No. 4,419,739 discloses a processor comprising several circuits, each of which has a local clock circuit of its own. Different clock circuits are controlled by a master clock, whereby the intention is to synchronize all circuits to the same frequency, which deviates very little from the frequency of the master clock. The solution is rather complex and becomes also rather expensive, because each circuit has a separate clock circuit. Further, synchronizing the clock circuits to the same frequency is fairly complex and difficult.
In publication US 2003/0 163 751, a clock signal is generated with an oscillator, and the intention is to transmit the clock signal to different destination points in such a way that there is as little signal delay and skew as possible. The solution presented is to use a low-loss transmission line together with a clock signal having a low rise rate. However, the system has a complex structure and thus the implementation on the whole is difficult, expensive and insecure.